Residual Stress Measurement and Forecasting System

It is well known that the fatigue properties of metals used in modern engines and aircraft structures are improved by local cold working processes, such as peening and cold working. These enhancements impart compressive residual stresses (RS) near and below the surface to inhibit or prevent crack initiation and propagation in critical components. The problem in aerospace systems is that the resultant RS is seldom measured during initial manufacturing or depot overhaul, and is further complicated because RS can relax significantly with operational usage. X-ray diffraction (XRD) technology affords a proven non-destructive means of acquiring quantitative surface RS information for use in diagnostics of aerospace system components. For prognostics, the Proto Team will conduct fatigue testing and RS measurements to augment its current life prediction methodologies. Empirically, RS relaxation has been found to occur primarily at or near the surface but may change beneath the surface layer. Proto plans to extend the working depth of its current XRD systems and provide both surface and at-depth RS measurements nondestructively, thus enhancing the output quality of its life prediction methodologies. Proto will develop a robust, portable and enhanced XRD system with prognostics software to support improved materiel management decisions and maintenance planning. BENEFIT: Proto will develop a non-destructive inspection system, or framework, utilizing x-ray diffraction (XRD) technologies for quantitatively measuring and evaluating the material state of residual stress (RS) in aerospace structures and engines, including cold worked areas around fastener holes. Protos portable systems provide a sound basis for in situ diagnostics during manufacturing and depot overhaul. XRD RSvery useful in prognostics enables evaluating the structural health and remaining life of individual components. Current XRD technology samples metals to a depth of roughly 10 to 30 microns, but Proto proposes to extend x-ray penetration much deeper. This advancement will provide surface and at-depth RS measurements nondestructively. Protos enhanced XRD system will enable robust and accurate life prediction based on RS depth profiles. The basis for x-ray enhancement will be established in Phase I. The resulting Phase II framework will provide OC-ALC, other depots and OEMs a means to measure in situ the RS state of manufactured structures and a sound means to: (1) Process control, test and verify design level of RS; (2) Manage product quality to RS; and (3) Make materiel disposition and life prediction decisions based on RS. Fundamentally, Protos proposed framework forms the basis for RS life-cycle managementa new paradigm.